When underground openings are created (e.g., tunnels, cavities, stopes, drifts, etc.) stresses are redistributed around the opening, which usually results in some ground/rock movement. If the movement is very sudden, a rock burst may occur. Convergence (i.e., squeezing) is the movement of rock gradually over time. FIGS. 1A-1C are diagrams showing three examples of how convergence often manifests itself in underground excavations.
In practice, mine or tunnel operators usually attempt to monitor (e.g., measure) ground movements in order to predict and/or avoid failures before they become unsafe. In many cases, convergence is not visible to the naked eye in the sense that tunnels may change in shape/move only millimetres or centimetres over long periods of time (e.g., weeks, months, or years).
Current techniques for monitoring convergence in underground excavations involve the installation of fixed infrastructure, such as a set of markers (e.g., reflective prisms). The positions of the markers are checked relative to each other by a systematic method for movement. Or, infrastructure may include the use of borehole extensometers, used to directly measure the movement of rock. Disadvantages of these existing methods include the fact that convergence is monitored only at specific and discrete locations, where infrastructure has been installed. This means that only select and few locations are monitored for convergence, rather than broad tunnel-wide or mine-wide coverage. Moreover, in some cases, measurements by existing methods can be laborious (i.e., a person must repeatedly return to manually capture measurements). Some instrumented extensometers are more automated for data collection, but they are costly and still suffer from the disadvantage noted above.